This invention relates to 1.times.N fiber optic couplers (N&gt;2) that are capable of coupling substantially equal amounts of power from an input optical fiber to three or more output optical fibers; it also relates to methods of making such couplers.
Methods of making 1.times.N couplers are disclosed in U.S. Pat. No. 5,017,206 and in U.S. patent application Ser. No. 07/913,390 (D. L. Weidman 6) filed Jul. 15, 1992, now U.S. Pat. No. 5,268,979.
Fiber optic couplers have been formed by assembling a coupler preform comprising two concentric glass tubes having a gap therebetween. The shape and/or size of the inner tube relative to the shape and/or size of the outer tube is such that a plurality of output optical fibers can be equally spaced in the gap. An input optical fiber is disposed in an bore in the inner tube. The input and output fibers extend through the midregion of the resultant coupler preform. The midregion is heated to collapse it about the fibers, and the central portion of the midregion is stretched to reduce the diameter thereof over a predetermined length.
The collapsing of the tube midregion causes the optical fibers just outside the collapsed region to be weakened. This weakened region occurs a few millimeters from the fully collapsed region in the uncollapsed portion of the bore of the outer tube. Pulling on the fiber pigtails extending from the coupler can cause fiber breakage. Moreover, under severe thermal cycling, the weakened region of the fibers has been known to break, primarily because of a thermal coefficient of expansion mismatch between the glue and the glass coupler components.
It has therefore been conventional practice to glue the ends of the outer tube to the fibers to provide those portions of the fibers that extend from the coupler with adequate pull strength. However, since the spacing between the input fiber and the inner glass tube is extremely small, glue applied to an end of the inner tube cannot flow between that tube and the input fiber. Therefore, an insufficient amount of glue contacts the input fiber to provide it adequate pull strength.